A wide range of new and efficient hydrogen-based power systems could be realized if the hydrogen fuel could be stored and released at temperatures and pressures consistent with the ambient operating conditions of the system. For example, military applications include hydrogen storage for stationary and mobile power sources, remote power, and low signature power; aerospace applications include hydrogen for auxiliary fuel cell power; automotive applications include hydrogen for fuel cell and combustion engines; commercial applications include hydrogen for stationary fuel cells for distributed power; and consumer applications include hydrogen for fuel cell powered portable electronic devices. However, it has proven difficult to provide local storage of hydrogen in a form that can be stored and released under moderate conditions.
Relatively low molecular weight metal hydride compounds, rich in hydrogen content, have been identified or synthesized. Such compounds can be viewed as storing hydrogen and thus available as candidates for supplying hydrogen to a hydrogen-powered device. However, the efficient operation of the device requires that the hydrogen is able to be easily and completely released from the compound on demand.
Currently, the most well developed hydride materials for reversible hydrogen storage can store about 7 wt. % hydrogen at ambient temperatures, but they require relatively high temperatures for hydrogen release. Typically such hydrides release most of their hydrogen content at temperatures of about 300° C., but they release only ˜5 wt % at 50-150° C., or only ˜2 wt. % at near ambient conditions. For example, magnesium hydride can reversibly store up to 7.6 wt. % hydrogen, but a temperature of 280° C. is required for an equilibrium pressure of 1 bar. This temperature is considered too high for most applications. Sodium alanate (NaAlH4) when appropriately catalyzed can yield up to 5.6 wt % hydrogen at temperatures of 50-150° C. Transition metal based materials such as TiFeH2 and LaNi5H6 yield only ˜2 wt. %, although the hydrogen can be evolved and recharged near room temperature.
The hydrogen storage materials described above are generally based on single-phase materials in both the hydrogenated and dehydrogenated states, ie, MgH2 and Mg, TiFeH2 and TiFe, and LaNi5H6 and LaNi5. Sodium alanate is an exception in which the dehydrogenated state is a two-phase mixture of NaH and Al.
It is an object of this invention to provide a group of two phase hydrogen systems comprising a hydrogen-containing lithium compound and a magnesium compound that permit the storage and release of hydrogen at more moderate conditions.